5 January 2017

STANAG-4538: an example of a 3G-ALE Asynchronous FLSU call

This recording is a real-world example of a 3G-HF FLSU (Fast Link Setup) asynchronous call copied on 15062 KHz/USB: although in 3G networks synchronous calls is the preferred mode, async call might be used if the called (or the caller) station may not have achieved net synchronisation. 
The async call of the fast link setup protocol begins with the LBT (listen before transmit) for at least one dwell period, followed by the transmission of 1.35N (nearest integer value) Async Request PDUs on the requested link frequency, where N is the number of channels in the scan list, and 1.35 is the duration of each dwell period in seconds. The async call  procedure ends with a single LFSU Request PDU (Fig. 1).

Fig. 1
The transmission copied on 15062 KHz/USB consists of several BW5 bursts (PDU_Request PDUs) sent multiple times (Fig. 2) followed by a single BW5 burst. Just the ending BW5 burst (FLSU_Term PDU) leads to think to a "1-way Async LQA Sound" scenario because a failed call will end with a xDL_EOM PDU (BW1 or BW4 bursts).

 
Fig. 2
Looking at the 50-bit payloads in Fig. 3, the PDU of type "011" is sent 13 times, followed by a single PDU of type "000": since PDU type "011" identifies the Async_FLSU_Req PDU, and "000" the FLSU_Request PDU, the sample exactly matches the async call procedure as illustrated above. It's worth noting that since there are 13 Async_ FLSU_Request PDUs, the number of the channels for this network is equal to 9.

Fig. 3
Quoting from STANAG-4538 "Transmitting 1.35N Async_FLSU_Request PDUs guarantees that all other scanning stations will scan the calling channel during the async call, even under the worstcase time of day (current time) offset conditions. If the time of day offset can be estimated more accurately, fewer than 1.35N Async_FLSU_Request PDUs may be sent to capture the desired station.
Since the address of the called station(s) is contained in the Async_FLSU_Req PDU, all stations that are not included in the call are free to resume scanning. Called station(s) that receive one of the asynchronous FLSU PDUs stop scanning and wait for the normal FLSU_Request PDU, which is sent immediately after the final Async_FLSU_Request PDU. The maximum wait duration is approximately equal to 1.35(N + 1) seconds, where N is the number of frequencies in the scan list". 

STANAG-4538 5.3 Annex C "Protocol data units" helps to parse the call:


001 00 0000100001 0000000100 0 0 011 111111 000010 10011000  [A
sync_FLSU_Req]
001 00 0000100001 0000000100 0 0 000 111111 000010 00001110  [FLSU_Request]

As a finale note, it's interesting to see in Fig. 5 that the on-the-air symbols of the async call have a 7296 bit length period, ie exactly 2432 PSK-8 symbols, just because of the (nearly) uniformity of the payloads.

Fig. 4
Fig. 5

In this recording, LP feature (Link Protecting) seems entirely disabled then PDUs are sent over the air unscrambled.

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